CODEWITHSUNDEEP
clinuxarraysunixshared-memory
RomainG
RomainG

Reputation: 597

C : shmget with a 2D array

I try to use shmget with a 2D array. This is my code :

char **array;
key_t key;
int size;
int shm_id;
int i = 0;
void *addr;

key = // here I get the key with ftok()
size = (21 * sizeof(char *)) + (21 * sizeof(char **));
shm_id = // here I get the shmid with shmget()

if (shm_id == -1) // Creation
{
     array = (char **)shmat(shm_id, NULL, SHM_R | SHM_W);
     while (i != 20)
     {
         array[i] = memset(array[i], ' ', 20);
         array[i][20] = '\0';
         i++;
     }
     array[i] = NULL;
     shm_id = // here I get the shmid with the flag IPC_CREAT to create the shared memory
     addr = shmat(shm_id, NULL, SHM_R | SHM_W);
}

But I've a segmentation fault with the line "array[i] = memset(array[i], ' ', 20);"

What am I doing wrong?

Upvotes: 1

Views: 826

Answers (1)

P.P
P.P

Reputation: 121407

You should first check whether shmget succeeds or not. if allocation of shared memory fails, then you can't be using shared memory! ;-) Like:

If ( shm_id = shmget(.......) == -1) {
exit(1);
}
else {
/* proceed with your work*/
}

and the same for shmat as well.

shmget returns void*. you can't assign it to a char** and and use it like a 2d array. In fact, a char* can easily be handled as a 2D array logically.

Upvotes: 2

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